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Flat Concrete Roofs

Problems with the deisgn and construction of flat concrete roofs

Reinforced flat concrete roofs must be carefully designed and properly constructed in order to avoid problems of poor drainage, cracking and leaks. Here we look at the problems, how they are caused and how to avoid them.

Flat concrete roofs

In Bali these days, in spite of the Balinese request that all buildings should have traditional sloping Balinese roofs, flat concrete roofs have become commonplace and, as it happens, leaks in flat concrete roofs have become almost as commonplace.

Standard design and method of construction

Most concrete roofs follow a similar type of construction. A 10 to 15 cm thick slab of reinforced concrete with a thin 2 to 5 cm thick layer of cement on the top. This cement layer is known as a "screed" and, contrary to popular belief, its purpose is not to waterproof the roof. It is, in fact, installed to provide a smooth sloping surface so the roof will drain correctly towards the drainage points thereby avoiding water "pooling" on the roof.

Flat concrete roofs as they are locally constructed have a number of issues.

Construction problems

The first thing to note is that it is the reinforced concrete slab, not the screed, that keeps the water out. It must be correctly designed and constructed. The walls must be built strong enough to support it. It should also be cast as a single piece of concrete with no joints in it. Any joints in the concrete may be sources of future leakage. The reinforcing steel must be of the correct size and should be correctly placed. It is also very important that the concrete is correctly mixed during construction, not too much water should be added and a vibrator should be used when pouring the concrete to remove any trapped air.

In addition the cement screed often causes problems. Under intense sunshine this thin layer can get very hot while the reinforced concrete of the roof beneath stays cooler. The screed expands more than the concrete and as a result the screed cracks and you will see cracks across the surface of the roof. This cracking also often results in the screed separating from the reinforced concrete beneath.

So now when it rains water can penetrate the cracks in the screed and can travel across the roof between the cement screed and the reinforced concrete and will manage to find any leaks through the reinforced concrete and into the room below.

Sealing leaks and waterproofing

When repairing a leaking roof the fundamental issue is that it is the reinforced concrete that is leaking and the best approach is to remove the screed and expose the reinforced concrete so that leaks can be found and properly sealed. The screeding can then be replaced.

This is a major undertaking and can seriously damage the roof if the people doing it don't know what they are doing. For example using jackhammers to remove the old screed can crack the concrete roof underneath making the problem worse.

We have been using specialist techniques for sealing concrete roofs by injecting the cracks. Such techniques can be carried out without removing the screed and have the added advantage of fully repairing the cracks effectively gluing the concrete back together again.

It is vital that correct methods and properly selected materials must be used.

Where do leaks occur?

Leaks are most likely to occur through cracks in the reinforced concrete or along joints where two distinct pours of concrete have occurred during construction. The cracks or joints may have opened up as a result of earthquake, ground subsidence, building settlement or possibly flexing of the roof slab either during or after construction.

It is important that the concrete of the roof is cast as a single reinforced slab. This slab forms an integral part of the building structure. The concrete or the reinforcing steel certainly should not be cut or damaged in any way as this can weaken the roof. It should be noted that if water is able to penetrate cracks in the reinforced concrete and comes into contact with steel reinforcing bars, the reinforcing bars may rust and this can cause serious problems further down the track. When steel rusts it expands and this can split the concrete apart.

Find the leak

Obviously we need to correctly identify precisely where the roof is leaking. It may be useful to expose the roof slab from underneath by removing part of the ceiling below to more accurately identify the point of water penetration.

Once found the leak must be fully sealed. This should be done from above - we wish to prevent water getting in. It is very difficult to effectively seal it from below.

Attempting to seal the screed is not easy. One crack may be all that is needed to allow water to penetrate under the screeding and from there may be able to reach large areas of the roof underneath. The application of a wash of cement based sealant over the surface of the screed is unlikely to be successful and may create more problems than it solves. Cement based sealants tend to be brittle and when exposed to excessive temperature fluctuations may crack or spall off the screed.


A membrane system may be used over the top of the screed. Rubber or plastic sheeting are usually a total disaster and should be avoided. Hot tar (bitumen) based systems can be very effective.

There are other waterproofing systems available that may work but most require correct application with attention paid to detail, this is of particular importance if sophisticated modern materials and techniques are to be used.

At the end of the day everything depends upon the contractor having sufficient knowledge and skill to correctly find the points of water penetration and seal them effectively.

Other solutions if the roof cannot be sealed

So you have a flat roof that can't be sealed, you have torn your hair out, shouted at the missus and kicked the cat, now what can I do? Well you may consider going back to basics and installing a pitched roof over the top. There are several options available from fully pitched tiled roofs to lower profile roofs using zinc alume or Onduline sheeting.

In Bali there is a notable gap in the range of building contractors available. There are large professional construction companies at one end of the scale and small private builders at the other with few medium scale operators in between. The large contractors are generally very capable however, as we have said, the vast majority of small builders simply do not have the understanding or the supervisory capacity to build reliable flat concrete roofs.

It is probably best to avoid flat concrete roofs in the first place. Traditional roofs are cheaper, more effective and well within the capabilities of local builders. They are also a lot cooler than pitched roofs which collect a lot of heat which is transferred into the house beneath.

Copyright © Phil Wilson March 2016
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